1. Field of the Invention
The present invention generally relates to two-way radio communication systems. More specifically, the present invention pertains to maintaining bit synchronization of a digital data stream transmitted over a radio frequency (RF) channel in a mobile radiotelephone system.
2. Description of the Prior Art
One general goal of mobile radiotelephone systems is to provide system features corresponding to those of the landline telephone systems to which they interconnect. One such feature is the transmission of data from one location to another. Many telephone subscribers connect data communications devices, such as computer terminals, to the telephone system via a modem. Basically, modems convert the digital data to distinct tones or tone/phase relationships which can be transmitted by the landline telephone network. Accordingly, a desired feature would be to have the ability to connect such devices, via a modem, through the landline telephone network, to a radiotelephone subscriber unit having a similar data communications device.
Those skilled in the radio art will appreciate the reasons why previous attempts to provide this interconnection have yielded unsatisfactory results. Rapid multipath fading, commonly experienced in high frequency radiotelephone communications, causes significant amplitude and phase changes in modem-generated tones such that data carried by the radio channel becomes garbled or missing.
Furthermore, system signalling requirements, e.g., a handoff between cells of a cellular radiotelephone system (which is virtually unnoticeable during a voice conversation), becomes a formidable obstacle during data communications. This particular limitation has been resolved by converting the input data to a format compatible with radio channel transmission, and reconverting the radio channel data back to the original format upon reception. The data transmission is halted prior to a handoff and is resumed after the handoff is completed. A detailed descripTion of the above process may be found in U.S. Pat. No. 4,654,867 by Labedz, et al. and assigned to the assignee of the present invention.
A second feature of landline telephone systems which subscribers of a radiotelephone system desire is that of security of their conversations. Digital scrambling techniques, which render a message unintelligible to prevent unauthorized reception, have been disclosed in U.S. Pat. Nos. 4,167,700; 4,434,323; and 4,440,976; each assigned to the assignee of the present invention. Typically, a voice message is digitized and processed through an encryption device to produce a signal that is random or pseudo-random in nature, thus appearing noise-like to the unauthorized receiver The particular encryption algorithm used by the encryption device may be a proprietary algorithm, or may be based on a Government standard (e.g. D.E.S.).
Additional obstacles must be overcome in order to accomplish secure voice communications via digital encryption of speech in a radiotelephone system. First, the aforementioned signalling requirements of the radiotelephone system must be addressed. This typically translates into additional overhead data words that must be transmitted concurrently with the digital voice signals. For example, digital speech at 9.6 kilobits per second (kbps) may be combined at the mobile unit with 400 bps signalling information and transmitted over the RF channel at a 10 kbps data rate, as described in the aforementioned related applications. The base site unit removes the signalling word and sends only the 9.6 kbps digital voice to the landline user. A standard 9.6 kbps modem can then be used to transmit the data over the telephone line, which appears to solve the problem.
However, the decryption of digitally-encrypted speech presents additional problems. It is generally known to operate an encryption device in a cipher feedback (CFB) mode, wherein the encrypted text output signal is added (modulo 2) with a plain text input signal. Those skilled in the art will appreciate that the wide utilization of the CFB-mode of operation was due chiefly to the self-synchronizing nature of the signal. However, CFB-mode devices operating over an RF link suffer a serious detriment in that the receiving units have a reduced operational range. The range reduction is caused by error multiplication in the CFB descrambler associated with receiver sensitivity. The error multiplication is fundamental to the CFB concept, since an erroneously-received bit is is fed back to the decryption device input until the error finally "clears" or propagates out.
Numerous bit errors are introduced by Rayleigh fades, which are caused by the mobile unit traveling through the multipath interference patterns generated by reflection from obstacles near the transmitting equipment. Hence, the receiving unit must remain within a given proximity of the transmitting unit in order to maintain the high signal-to-noise ratio necessary to preclude receiving erroneous data bits. Although the CFB descrambler will eventually re-synchronize itself, the numerous errors in the output text stream could mean a missed message or faulty data.
A second scrambling technique, the counter address (CA) mode of encryption, does not suffer from the error multiplication problem characteristic of the CFB-mode operation. "Counter addressing" means that the device implementing the selected encryption algorithm is fed a pseudo-random sequence to generate the key stream, which is used to encrypt the plain text message. In this way, the encryption device is "addressed" by the pseudo-random counter. However, continuous bit synchronization from the scrambler to the descrambler is required to allow proper operation of the CA-mode descrambler key generator without requiring periodic key generator data transfers. Continuous bit synchronization, as used herein, means that the number of bits into the transmission channel is equal to the number of bits recovered from the channel, and that the average data rate into the channel is equal to the average data rate out of the channel.
Bit synchronization over an RF transmission path is very difficult to maintain, primarily due to Rayleigh fading. Any method of continuous bit synchronization must withstand RF phase jumps, channel fading, drift between RF and landline modem clocks, and bit slippage due to an out-of-lock phase-locked loop (PLL) at the mobile or base site.
Known techniques for maintaining bit synchronization involve a series of tradeoffs when used over an RF channel. One method is to utilize additional overhead bits toperiodically re-initialize th pseudo-random number generators of the scrambling devices. However, the use of any additional overhead means that a substantial penalty is paid for the reduced data rate of the voice signal as compared to that which is possible on a fade-free channel. If radiotelephone system signalling overhead is also required, the further reduction in overall data rate would significantly degrade the quality of the digitized speech. Conversely, CFB-mode encryption may be used with its error multiplication and range reduction limitations.
A further problem involves maintaining bit synchronization through standard modem which typically have tight specifications with respect to clock and data input tolerances. For example, a standard V.32 modem requires a 9.6 K.+-.100 parts-per-million (ppm) input data rate. Hence, any differences between the mobile unit's data transmission clock frequency and the modem clock used at the base site must be compesated for, while still maintaining the bit integrity of the channel. Furthermore, when any overhead signalling words are removed from the data stream, the clock and data must be buffered for transmission by the modem--again while maintaining continuous bit synchronization.
A need, therefore, exists to provide a way to maintain continuous bit synchronization in an RF channel, from the mobile unit through the base site to the landline unit, while remaining compatible with standard landline data transmission equipment.